1. Field of the Invention
The present invention relates to an apparatus for predetermined and reproducible formation of doping material in a semiconductor crystalline rod, particularly a silicon monocrystalline rod, by nuclear reactions.
2. Prior Art
Silicon rods having a predetermined dopant concentration are required for the production of high quality semiconductor components. During the production of such doped rods, the desired dopant concentration must be distributed as homogeneously as possible over the volume of the rod, both in the radial and axial directions.
Various processes are known which seek to provide homogeneously doped semiconductor rods. Generally, the doping of a semiconductor rod is conducted during the deposition of a semiconductor material from a gas phase thereof by thermal decomposition of a suitable gaseous compound yielding the semiconductor material on a heated carrier body or mandrel. The gaseous compound of the desired dopant material is mixed with the gaseous compound of the semiconductor material and simultaneously decomposes and the dopant and semiconductor material are simultaneously precipitated on the carrier body. A resultant crystalline rod produced in this manner is polycrystalline in nature and must be converted into the monocrystalline state by a subsequent zone melting process. During this process, the dopant concentration often changes in an uncontrollable manner and substantially higher (than actually desired) initial doping concentrations must be used to insure that the desired dopant concentration is still contained in the final monocrystalline product, which may require a plurality of zone melting cycles. Thus, in the zone melt process, the polycrystalline rod must be doped during the deposition process directly from the gas phase or via a highly doped core in such a manner that a desired specific resistance or specific recombination center density is achieved over the entire rod length, with proper compensations for the zone pulling and evaporation effects. However, it is not possible to achieve high concentration of dopant substances in the finished monocrystalline rod in this manner. Further, high concentrations of dopant substances close to the solubility limit (10.sup.19 14 10.sup.20 cm.sup.-3) cannot be obtained by doping from a gas phase process wherein a gas stream charged with a dopant is blown onto a molten zone. In conventional methods, it is also difficult to produce lower dopant concentrations, for example, 10.sup.13 -10.sup.14 cm.sup.-3 in an accurate and reproducible manner.
Generally, the prior art apparatus used for practicing the above-described processes comprises an evacuable hollow container or housing in which a semiconductor crystalline rod is supported vertically by rod holders extending through the top and bottom walls of such a container. A select dopant is either mixed with a gaseous semiconductor compound in a carrier gas stream and blown onto the melt zone of the rod, which has been heated by the passage of electric current so that the dopant and semiconductor compound thermally decompose on the rod (C-process) or the dopant is directly fed to the molten zone by means of a carrier gas stream during a crucible-free zone melting process. In the latter process, boron and phosphorus compounds which are easy to handle and evaporate at relatively low temperatures are typically used as dopants. The doping dosage is typically controlled by valve means.
A substantial disadvantage of such apparatus is that the valve means used for the dosage control of the dopants fail to act in an accurate and reproducible manner. Of course, this effects the reproducibility of the dopant concentration in a crystalline silicon rod produced in this manner. Further, such processes and apparatus used to conduct them produce a more or less inhomogeneous distribution of dopants after the zone melt defining procedure.